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LTC4224CMS-1-PBF 数据表(PDF) 12 Page - Linear Technology |
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LTC4224CMS-1-PBF 数据表(HTML) 12 Page - Linear Technology |
12 / 16 page LTC4224-1/LTC4224-2 12 422412fa APPLICATIONS INFORMATION Next, assume that there is no load current at start-up, and calculate the inrush current required to charge the load capacitor. As there is no gate capacitor, the supplies start-up in current limit. Compute the time, tSU, it takes to fully charge the load capacitor: tSU = VCC •CLOAD ITRIP Table 3 lists the worst-case tSU values assuming 30% tolerance for load capacitances. Table 3. Worst-Case tSU VOLTAGE SUPPLY tSU(MIN) tSU(MAX) 5V 0.53ms 0.65ms 3.3V 0.23ms 0.29ms The start-up ECB blanking delay is guaranteed to be at least 2.5ms, which is longer than the tSU tabulated in Table 3. Hence, both supplies can start up successfully. Next, verify that the thermal ratings of the selected external MOSFETs are not exceeded during power-up or an output short-circuit. Assuming the MOSFET dissipates power only due to inrush current charging the load capacitor, the energy dissipated in the MOSFET during power-up is the same as that stored in the load capacitor after power-up. The average power dissipated in the MOSFET is given by: PAVG = CLOAD •VOUT 2 2• tSU The worst-case PAVGiscalculatedtobe4.6Wforboththe5V supply and the 3.3V supply. In this example, the FDS6911 MOSFET offers a good solution. Since this MOSFET is a dual N-channel in a single SO8 package, it must be able to tolerate the combined power dissipation of both supplies during the tSU start-up time. The increase in steady-state junction temperature due to power dissipated in the MOSFET is ΔT = PAVG•ZTH where ZTH is the thermal impedance. Under this condition, the FDS6911 datasheet’s Transient Thermal Impedance plot indicates that the junction tem- perature will increase by 6.4°C using ZTHJC = 0.7°C/W (single pulse). The FDS6911’s on-resistance is 17mΩ at VGS = 4.5V, 25°C. The magnitude of the power pulse that results during a severe overload is calculated to be 9.25W for the 5V sup- ply and 9.2W for the 3.3V supply under the worst case conditions. Assuming a worst-case circuit breaker timeout period of 7.5ms, the junction temperature will increase by 25°C, with one supply short-circuited. If both supplies are short-circuited, the junction temperature will increase by 50°C in the worst-case. During auto-retry (LTC4224-2), in the event of persistent faults at both supplies, the ample four second cooling delay limits the increase in junction temperature to 50°C. The SOA curves of the FDS6911 indicate that the above conditions are safe. |
类似零件编号 - LTC4224CMS-1-PBF |
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类似说明 - LTC4224CMS-1-PBF |
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